Radio locating apparatus



mec. 28,1948. P, E POLLARD 12,457,396

y RADIO LcATlNG APPARATUS l y Filed 00j.. 8, 1943 10 Sheets-Sheet 1 [YY Il I OVF Pyr i xIn/genior i Byfoyd.

Attorney Dec. 28, 1948.v P. E; POLLARD RADIO LOCATING APPARATUS 10 Sheets-Sheet 2 Filed Oct. 8, 1943 WENT/METER MQVEMENT TIME ec. 28, 194. P. E. PQLLARD 2,457,395

RADIO LOCATING' PPRATUS Filed oct. 8,A 194; 1o shets-sheet 3 Dec. 28, 1948. P. E. .POLL/ARD RADIO LOCATING APPARATUS 10 Sheets-Sheet 4 Filed oct. 8, 194:5

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Attorney De@ 23 1948- P. E. POLLARD RADIO LOCTING' APPARATUS 10 Sheets-Sheet 5 Filed Oct. 8, 1943 P. E. PQLLARD RADIO LocATING APPARATUS Deco 28, 1948.

Filed oct. 8, 194sv l0 Sheets-Sheet 6 P a lefr-mening Mil ,1j-Lamm.

A Homey P. E. POLLARD RADIO' LocA'rING APPARATUS Dec. 28, 1948.

Filed oct. s, 194s 10 Sheets-Shea# 7 mwN Dec. 28, 1948. P E POLLARD 2,457,396

RADIO LOCATING APPARATUS Filed Oct. '8. 1943 10 Sheets-Sheet 8 Puf am@ By f1@ Mlm,

A ttorney Dec. 28, 1948. P. E. POLLARD 294573396 RADIO LOCATING APPARATUS Filed oct. e, 1943 10 Sheets-Sheet 9 M n ...A

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Inventor Attorney Dec. 28, 1948.

Filed Oct. 8. 1943 P. E. POLLARD RADIO LOCATING APPARATUS l By fovf Inventor PLA-f and mmf Mlm.

Attorney Patented Dee. 2%, i948 gisant rica y anni@ rosarino arraaarns Philip Edward 4iholllalrail, London, England, assignor to Minister oi Supply in His Maiestys Government oi the United Kingdom ci Great Britain and Northern ireland, London, England Application October 8,1943, Serial No. 505,553

lin Great Britain March 2a, 1942 Section l, Public Law i390, August ldt Patent expires March 24, 1962 The present invention relates to radio apparatus for locating the position and following the movement of a distant body by means of radio irequency energy radiated in the form of an exploring beam by a transmitter and reflected by the target back to a receiver.

lin the apparatus according to the invention the exploring beam is pulse-modulated; that is to say, it consists of very short pulses of radio-frequency energy separated by much longer quiescent periods during which the reflected pulses or echoes can be received. By means of a cathode ray tube oscillograph the time interval between the transmission oi an outgoing pulse and the receipt of the incoming echo is measured, this time interval being a measure of the slant-range of the body giving rise to the echo. The echosignal is also picked up on an aerial system having a sharp directional response and by means of a second cathode ray oscillograph the amplitude of the echo signal picked up on thisv aerial system can be observed for the purpose of determining the angle of bearing of the body. Another aerial system and associated oscillograph can be employed in a similar manner for determining the angle of elevation of the body.

One object of the present invention is to provide improved means for measuring the time interval between the outgoing and incoming signals.

Another object of the invention is to provide means for correlating the presentation of a. selected echo signal on the several osclllographs so that the several observers of these oscillographs can make their simultaneous observations on one and the same target without ambiguity or confusion.

Another object of the invention is to provide improved means for determining when the orientation of the aerial system is such that the ani-v plitude of the echo signal picked up by it is a minimum, and for determining without ambiguity whether changes in this amplitude indicate increases or decreases in the angle of bearing (or elevation) of the body.

Another object of the invention is to provide improved means for generating very short pulses of radio-frequency energy at high power.

Further objects of the invention will appear from the following detailed description of two alternative forms of apparatus embodying the in- 7 Claims. (Ci. 3dS-lll) vention. The lrstyform of the apparatus is described with'reference to Figs. 1-11 of the accompanying drawings in which Figs. 1 and 2 are block diagrams of the trans- Y mitter and receiver;

Fig. 3 is an explanatory diagram;

Figs. 4 Iand 5 are circuit diagrams illustrating the method of potentiometer control;

Figs. 6 and 7 are circuit diagrams of the modulator and master oscillator;

Fig. 8 is a circuit diagram of the signal re- Fig. 9 is a circuit diagram of the time-base circuit;

Fig. l0 is a circuit diagram of the calibrator unit; and

Fig. 11 is a diagram showing the layout of the cathode ray tubes and associatedvcircuits.

The second form of apparatus is described with reference to Figs. 12-19 of the drawing in which Figs. 12 and 13 are circuit diagrams of the master oscillator and modulator unit;

Fig, 14 isa block diagram of the receiver;

Figs. 15 and 16 are circuit diagrams of the signal receiver:

Fig. 17 (a, b, c) are schematic diagrams of the time base generators and associated potentiometers; l

Fig. 18 is a circuit diagram of the time base circuit; and

Fig. 19 is a circuit diagram of the signal selecting and automatic gain control units.

The apparatus to be described with reference to Figs. 1-11 of the drawings is designed to provide a continuous indication of distance or slant range of a target by means of a cathode ray tube and-a separate indication of the angle of bearing of the'target on al second cathode ray tube and is provided with means whereby both indications receiver.

The transmitter comprises a master-oscillator i operating on a wavelengthof between 3.5 and 5.5I metres connected through an amplifier 2 to a half-wave di-pole aerial 3. The oscillator is normally blocked by means of a negative bias, but this is periodically removed by the application of positive voltage impulses from the modulator unit '4, the frequency of these impulses lying within the range of LUGO-2,500 per second. The master-oscillator operates in the following manner: after being triggered by an impulse from the modulator 4 it generates a train of ultrashort wave signals until the resulting flow of rectiiied current builds up a prohibitive negative bias on the grid when it ceases until the arrival of the next positive impulse. As a result a pulsemodulated beam of radio frequency energy is radiated from the aerial 9L the duration of each pulse being 2 or 3 microseconds and the frequency of the pulses being from 100G-2500 per second. A portion of the output is tapped from the aerial feeder and passed through an attenuator to the signal plates of a monitor tube the sweep plates of which are fed with a sweep voltage from the generator 'I which is locked to the modulator 4. Filament supply units for the oscillator I and amplifier 2 are indicated at 8, the H. T. supply unit at 9 and the grid bias unit for the amplifier 2 at I0, whilst II and I2 indicate the power units for the modulator and monitor tube.

If a target is present within the eld of the transmitter an echo-signal in the form of reected pulses of radio frequency energy will be received at the receiving unit shown in Fig. 2 which is situated in the neighbourhood of the transmitter. The receiver is provided with two aerial systems, one I3 consisting of a dipole provided with a keyed reflector (not shown) for determining the slant range of the target, and the other I4 consisting of a horizontally separated pair of dipoles; also fitted with reflectors, and coupled through a phasing-box I5, for determining the angle of bearing of the target. The output of the range Iaerial I3 is passed to a super heterodyne signal receiver consisting of a radio-frequency amplier I6, mixer I1, intermediate frequency ampliier I8, second detector I 9 and videofrequency output stage 20, Whilst the output of the bearing aerial I4 is passed to a second signal receiver consisting of a radio-frequency amplifier 2I, mixer 22, intermediate frequency amplifier 23 second detector 24 and video-frequency output stage 25. A single local oscillator 26 is common to both signal receivers and a buii'er stage 21 is included to prevent interaction between them. The output stage 20 is connected to the signal plates of the range indicating tube 28, whilst the output stage 25 is connected to the signal plates of the bearing indicating tube 29.

The transmitted signals are picked up on an aerial 30 the output of which is fed through a receiver 3l and amplifier 32 to trigger a multivibrator 33. The latter controls the time base generators 34, 35 which produce the synchronized sweep voltages for the indicating tubes 28, 29 and also provides positive voltage pulses to the grids of these tubes through the circuits 36, 31 for eliminating the back stroke. 38 and 39 indicate the power supply units for the indicating tubes 28, 29.

At the instant when each outgoing pulse leaves the transmitter, the multivibrator 33 is triggered through the receiver 3| and the sweep voltage applied to one of the sweep plates of the range indieating tube 28 from the generator 34 begins to increase. At a certain time later the incoming reilected pulse will arrive, this time depending upon the slant range of the target. The incoming echo will give rise to an echo trace on the screen of the tube 28, and clearly, the distance of this trace along the time base from the origin of the latter will be a measure of the slant range of the target giving rise to the echo. A similar trace will simultaneously appear on the screen of the bearing indicating tube 29, but since the outputs from the two spaced dipoles of the aerial I4 are connected in phase opposition through the phasing box I5, the amplitude of this trace will be a minimum when the bearing of the target coincides with the direction of minimum response of the aerial I4 which will be substantially in a direction at right angles to the line joining the two aerials. The bearing of the target is accordingly determined by swinging the aerial I4 until the amplitude of the echo trace on the tube 29 is a minimum.

The determination of the slant-range of the target could be done by direct measurement of the position of the echo-trace on a calibrated scale parallel to the time base on the screen of the tube 28. It would be Very difficult, however, to calibratel such a scale accurately in terms of the range of the object. This is chiefly due to the fact that in the more usual form of time-base generator, the forward sweep of the spot is controlled by the sweep voltage built up across a condenser connected through a high resistance to a constant source of voltage. Consequently the sweep voltage increases exponentially and not linearly with time, so that direct calibration of the movement of the spot by means of such a scale becomes difficult, particularly towards the end of its sweep. Such direct calibration is further complicated by certain other disturbing factors which come increasingly into play as the spot moves away from the centreof the screen.

A second difficulty in operating the apparatus described so far arises from the'fact that there may be more than one target simultaneously present within the eld of the transmitter. As simultaneous observations of the screens of the tubes 28, 29 have to be made by different observers, and as there may very well be several echo traces corresponding to the several targets on the screen of each tube, it becomes very necessary to provide means whereby an echo-trace selected for measurement by the observer of the range tube 28 can be automatically correlated with its counterpart on the screen of the tube 29 so that both traces can be simultaneously identified with the particular target that has been selected for scrutiny.

These two difficulties are overcome by providing a range potentiometer 40 connected across the power` supply 38 and having its tapping connected to one of the sweep plates of the range indicating tube, and a similar potentiometer 4I connected across the power supply 39 and having its tapping connected to one of the sweep plates of the bearing indicating tube 29, both potentiometers being operated by a common control handle 42 under the control of the range tube observer. The function of these potentiometers and the manner in which they are designed will now be described with reference to Figs. 3-5.

Fig. 3 shows two echo traces a, b on the screen of the tube 28 due to the reflection from two targets, the displacement of each trace from the centre of the screen being a measure of the respective time intervals t1, t2, between the outgoing pulse and the echoes produced by reflection from the two targets. Instead of measuring such displacements directly on the screen of the tube, the range potentiometer 40 is used to bring any selected trace to the centre of the screen by applying a suitable biasing voltage to one of the sweep plates. It is clear that the required biasing voltage will be equal to the instantaneous value v1, vz of the voltage applied to the opposite deilecting plate by the time-base generator 34. The control voltage from the potentiometer 40 can therefore be used to calibrate the movement of the spot or trace.- Curve I shows sweep-voltage plotted against time, and curve II shows potentiometer voltage plotted against the movement of its variable control, instantaneous values being indicated for the two echo-traces a, b. It is required to maintain a linear relation between any given time-interval such as t1, 'tz and the correspon fifa movement 0l., 92 of the potentiometer contrai.

In Fig. 4, the sweep plate 43 of the range-indicating tube 2B is supplied with a sweep voltage from the exponential time-base 34 comprising a condenser 44 connected to a constant source vof voltage Vo through a charging resistance 45. The condenser 44 is periodically discharged at the end of each sweep by the automatic triggering of a value 46 so as to bring the spot back to its initial or zero position. The potentiometer 40 is branched across the vsame voltage source Vo, and Athe control or centering voltage tapped off by a slider 41 is applied to the opposite plate 48 of the tube.

The voltage V on the time-base condenser 44 at any given moment t is equal to Vo. (l-e-i/CR) where Vo is the voltage of the supply; C is the capacity of the condenser 44; and R the value of the charging resistance 45. If the potentiometer slider 41 is set to centre a selected trace, the value of the restoring voltage will be R2/R1 Vo, where R1 is the total value of the potentiometer resistance and R2 that part of it which is tapped oi by the slider. For any given setting of the slider, R2/R1 is constant and can be replaced byJc so that Vo(1-e*f/CR) :K Vo, and (1-e-t/CR) is constant, and independent of any variations that may occur in thesupply voltage or in the Working characteristics of the tube.

When a selected trace has been centered, the voltage on plate 43 being equal to the voltage on plate 4B, there is no deecting field, and the centre of the screen is the electrical zero of the systern, This gives optimum focussing of the echotrace and sweep-line and, is general, the most favourable conditions for observation. Moreover the balanced method of control permits the use of a cathode ray tube of relatively small dimensions, since any necessary adjustments for critical observation occur at the centre of the screen. For operational reasons the potential of both plates 43, 48 should be approximately the same as thatl ofthe third anode, which is earthed.

The time-constant upon which the grading or calibration of the potentiometer 40 is based is,-in practice, determined (a) by the value of the timebase voltage necessary to drive the spot at the required speed, (b) by the values of the charging and potentiometer resistances required to keep the current within reasonable limits, and, after these two factors have been xed, (c) by the particular value of capacity which in combination with the charging resistance 45 is found to give the most favourable form of curve over the range selected for calibration.

A. time-interval of six microseconds corresponds approximately to a slant range of 1,000 yards. Taking Vo=4,000; Rrl megohm; R1=2 meg-- ohms; and a time-interval of 6X14=84 microseconds. to cover a slant-range of 14,000 yards, the

y time-constant CR for the most favourable form of curve was found to be 1/6000. This sets the value of C at 160 m. mf. l 1

At a given time tthe sweep voltage on the deecting plate is V=Vo (l-e-f/CR). Considering the current I then flowing through the potentiometer, V=R2I and Vo=R1I. Therefore by substitution R2=R1 (l-rf/CR) A selected part Rz of the potentiometer resistance can then be calibrated say in slant-range units of 50 yards, by giving corresponding values to t in the above expression. If as before, 6 microseconds represents a slant-range of 1,000

yards, the equivalent time for 50 yards will be 0.3

microseconds, and the calibration formula becomes 2x10, (1-e-18). Resistance bobbins each representing a 50-yard unit, are then separately wound and connected to contacts arranged at equal intervals around the potentiometer dial, so that equal angular movements of the control slider 41 represent equal distances in slant range.

Calibration on this scale can be extended as far as required. As a matter of expediency, the 50- v yard unit is applied in one form of apparatus to ranges between 2000 and 14,000 yards, the first 2000 yards being ignored. A calibration unit of say 250 yards can then be used for the longer ranges of operation, say betweenV 14,000 and 30,000 yards, by successively varying the overall resistance left in series with the voltage supply thus further increasing the voltage on the slider. In this case calibration is most easily effected by trial and error. A stationary wave-form, representing say a six microsecond interval, is projected on to the screen of the tube by a crystal-controlled oscillator, and the value of resistance required to shift a selected wave through the equivalent of each 250 yards of slant-range is determined bylactual test. Corresponding resistance bobbins are then wound and connected as before, to a series of contacts on a second dial which is arranged to come automatically into action when the rst or 50-yard dial reaches its maximum setting of 14,000 yards.

Fig. 5 illustrates how the second difficulty mentioned above is overcome by using two ganged potentiometers 40, 4I. The time-base generator of the range indicating tube 28 is fed .from a 4000 volt supply 38 and is of the exponential type comprising a condenser 44, charging resistance 45, and triggering valve 46. The time-base generator of the bearing indicating tube 29 is similarly fed from a 2000-volt supply 39 through resistance 49, condenser 50 and triggering valve 5l, the values of resistance and capacity being based upon the same time-constant in both cases. The bearing control potentiometer 4i is electrically symmetrical with the range control potentiometer 40, and the earthed slider 52 is ganged to the sliders 41 and 53. It follows that when any selected echotrace is brought by the slider 41 tothe centre of therange tube 28 for observation, the counterpart of the same trace will be simultaneously centered on the screen of the bearing tube 29. This correlation automatically identies both traces as `being due to the particular target selected for observation. The screens of the tubes are preferably provided with suitable reference marks X to enable accurate centering to be achieved.

For reasons already stated, the voltage on the sweep-plates should be approximately the same as the voltage on the earthed anode of the tube. The potentiometer 40, the time-base circuit 44, 45, 46, and the 400G-volt power-pack 38 are accordingly all carefully insulated, and the control aisasoe parallel with the potentiometer 40 and would vary the current ilowing through it. To limit the error due to this cause to less than one in a thousand, particularly at the higher-range settings, the leakage resistance should be kept above 1000 'megohma which is a standard oi' insulation dilcult to attain in practice.

Accordingly, instead of directly earthlng the control slider 41, the potentiometer 40 is connected to a parallel resistance 54, which serves as a leakage guard. The resistance 54 is electrically symmetrical with the main potentiometer 40, and is directly earthed through a slider 53v which is ganged to the control slider 41. Unavoidable leakage currents now pass to earth through the guard without varying the current through the main potentiometer 40. A slight potential diilerence may be set up between the'sliders 41 and 53, but this can be ignored. provided it is not sufficient to upset the focussing of the spot. The provision of the guard reduces the required degree of insulation to practicable limits without involving a range error of more than one in a thousand. The similar leakage guard is not provided for the bearing potentiometer 4I as the same standard of accuracy is not required from the bearing tube.

As previously mentioned the first 2000 yards are ignored, this being covered by the resistance 55 which is therefore not calibrated. Also the remaining resistance 56 in series with the potentiometer 41 may be separately calibrated and controlled in steps of 250 yards and used for ranges between 14,000 and 30,000 yards. Resistances 60, 6I corresponding to the resistance 55, and variable resistances 62, 63 corresponding to the variable resistance 5B and ganged with it, are provided in series with the potentiometers 4I and 54.

The normal line-sweep across the screen is suicient to include echoes from bodies within a range of 14,000 yards. It is, however, possible, by switching a resistance 58 into series with the tima-base supply, to reduce the voltage across the X-plates of the tube 28 so as to bring echoes from bodies within a distance of say 30,000 yards on to the screen. An "overrun pointer, mounted in front of the range screen, is then brought into line with a selected echo, and the correspending range is read E from the calibrated scale on which the pointer is based. A corresponding overrun pointer on the bearing tube 29 can then be adjusted to the same reading and a bearing taken of the echo-trace so identified. It will be noted that these operations serve as a preliminary to the use of the potentiometer control lor making more accurate observations. Bearing signals are received on the pair of horizontallyspaced dipoles I4 provided with reflectors, the pick-up energy being opposed in the phasing box I5 so as to give a directional response with two broad and two sharp minima. Bearings are taken on the sharp minima, which is prearranged to be in line with the maximum response of the range dipole I3.

When an echo trace has been centred on the screen of the range tube 28, and its "sense has been determined, the bearing oi the body under observation is taken by swinging the directional aerial system I4 until the echo trace then at the centre of the bearing tube 28 shows at minimum amplitude. The necessary rotation of the aerial! is effected by means oi a pair of hand-wheels mounted on a. traversing column or post, which is iltted with a dial to indicate its orientation.

The movements of this traversing column, as well as those of the range potentiometer control, may be transmitted through Selsyns or similar synchronizing gear to a distant control point.

A more detailed description of the various circuits employed in the transmitter and receiver will now be given with reference to Figs. 6-11.

Fig. 6 shows the modulator unit 4 of the transmitter in detail. A strongly back-coupledyalve generates a low-frequency relaxationl voltage which controls the pulsing or repetition frequency of the master-oscillator I (Fig. l). The repetition-frequency can be varied within limits say of 1000 and 2500 cycles, either to minimise interference or for other reasons, by adjusting a resistance 66 shunted across the grid circuit. The valve 65 is coupled through amplifiers 61, 88, which also serve to peak the voltage-wave, to the grids of two cathode-followers 69, 10, the cathodes of which are normally held at about 900 volts negative. The onset of the control pulse throws the grids oi 69, 10 positive, the cathodes follow suit, and a corresponding positive pulse is fed through a small condenser 1I to the grid Lecher wires of the oscillator unit I. A diode valve 12 is provided to bypass any reverse gridcurrent in the oscillator, whilst a second diode 13 provides a safeguard against a flash-over.

The ampliiier 61 is also coupled to a pentode 14 which serves as the discharge valve in the sweep voltage generator 'I of the monitor cathode ray tube 6 (Fig. l). Pulses from the cathode of the valve 69 are also applied through the condenser 15 to the negatively biased grid of the monitor tube 6 to release the electron stream during the forward movement of the spot over the screen; this, in effect, serves to eliminate the back-stroke, and gives a clearer picture.

The master-oscillator stage I for generating the pulsed high-frequency signal, is shown in detail in Fig. 7. It comprises two push-pull valves 16, 11, the anode and grid circuits being tuned by Lecher wires 18, I9 over a range of from 3.5 to 5.5 metres. A paralysing negative bias is normally applied to the mid-point of the common Lacher-wire connection to the two grids. When this is removed by the low-frequency pulsing voltage applied through condenser 1| the oscillator is triggered into operation, and continues to generate a train of ultra-short signal waves until the resulting ilow of rectified current builds up a prohibitive negative bias on the grid. HyA suitably selecting the time-constant oi' the grid circuit, this action is utilized to limit the duration of each train of signal waves to a period of from 2 to 3 microseconds after which radiation ceases until it is again initiated by the application of the next'pulsing-control impulse f-rom to prevent regenerative feed-back, and the grids are normally biased to cut-off.

The oscillator and radio amplifier circuits are provided with vsuitable tuning controls 89, 90'and The transmitting aerial 3 is a telescopic halfwave dipole mounted to rotatel through approximately ninety degrees. It is coupled, through telescopic matching stubs and a feed-line, to one or the other of two sets of terminals on a changeover switch 92 which is ganged to the wavechange 'switch used for the pre-set frequencies.

The aerial can be used at two different heights, the higher setting being preferably used for the longer waves, and the lower for the shorter. Two blocking condensers 93, 94 between the couplingtaps to the amplifier and the change-over switch allow the aerial to be earthed through a radiofrequency choke 9E.

The circuits of the signal receiver Iii-2,1 (Fig. 2) are shown in detail in Fig. 8. The incoming signals from the aerial I3 are first passed through a four-stage radio-frequency yamplifier I6, of which the first two stages and |0| are shown in Fig. 8, the output from the fourth stage (not shown) being fed to a hexode mixer I1. This, in turn, is coupled to 'a common local-oscillator stage 26, (of which Ithe first harmonic is used) through a bufer" valve 21, which is included to prevent interaction and the transfer of noise from the range to the bearing channel. 'Ihe mixer feeds a four-stage intermediate-frequency amplifier I8 of which the first two stages are shown at |02 and |03. The output from the fourth stage (not shown) is passed through a, diode rectifier I9 to an output valve 20, feeding one ofthe Y-plates of the range tube 28.

Signals from the bearing aerial I4 pass through a similar four-stage radio frequency amplifier 2| (the first two stages |04, only .being shown in the drawing) to a mixer 22, which is coupled directly to the oscillator 26. From the mixer 22, the signals pass through a separate intermediatefrequency channel 23, 24 to an output valve 25 feeding one of .the Y-plates of the bearing tube 29. To save space, this channel 23-25 is not pent-odes 4-6, 46A which operate in parallel to discharge the timing condenser 44. The input ampliiiers I I0, are triggered by the outgoing sigshown in Fig. 8, as it is identical with the channel I8-20 for the range signals.

The input circuit of each of the first radio-frequency amplifier stages |00 and |04 includes a semi-aperiodic transformer |06, |01 designed to resonate with the valve capacities at ya frequenCY in the middle of the lower Wave-range. On the higher range, series condensers |08, |09 are switched into circuit. 'Ihe remaining stages are tuned by variable inductances with powderediron cores which are ganged to a common controlcam.

The local-oscillator stage 2B is-tuned by a separate cam which is shaped -to ensure correct tracking over the whole range. To avoid having to switch this stage when changing from one wave-range t-o the other, the expedient is adopted of using for the longer-wave range an oscillation frequency which is higher than the original frequency and for the shorter-wave range an oscillation frequency which is lower than the' signal frequency. The intermediate frequency is fixed at 7.7 megacycles, though the ,tuning of'alternative I. F. stages is staggered, one slightly above and the other slightly below the fixed value in order to cover a wider band. For this reason the tuning of each I. F. stage is individually controlled.

Circuit details of the amplifier 32, multivibranal from the transmitter, through the locking receiver 3| (Fig. 2). This puts a high negative impulse on the grids of the pentodes 4646A so that the timing condenser 44 starts to charge. Its potential is applied to one of the X-plates of the tube 28, and the spot begins to move across the fluorescent screen. s

The voltage drop across the anode resistance ||4 ofthe -amplifier I I I has meanwhile reversed the initial setting of the cross-coupled unit |I2 and ||3 so that I3 now passes full anode current whilst ||2 is blocked. This condition lasts for a period determined by the time-constant of the condenser ||5 and resistance IIB, and serves to hold the grids of the valves 46 and 46A at cut off for a period of about 300 microseconds. The lrelaxation unit then returns to its initial setting, and the timing condenser v44 is discharged. A quiescent period follows. until the arrival of the next impulse from the locking receiver.

A lead from the relaxation unit ||2, II3 is taken through the condenser I|'| to the grid of the discharger valve 5I ofthe time base generator 35 (Fig. 2) for the bearing tube. This circuit is not shown infdetail as it is substantially similar to that of the range tube.

The valves 36 and Sl are also energized by the arrival of each locking impulse, and apply a positive pulse to the normally over-biased grids of the range and bearing tubes, so as to release the electron stream during the forward traverse of each spot. In effect, this eliminates lthe back stroke. and helps to give a clearer picture.

A calibrating unit |20 (Fig. 2) for providing an artificial echo signal of known characteristics can be connected to the signal plates of the range tube 28 through the switch |2|. The details of this unit are shown in Fig. 10. It includes a selfstarting crystal-controlled oscillator |22 which generates waves at a fixed frequency of 164 kilocycles, corresponding to a time-interval of 6 microseconds, or a range of 1000 yards. A strongly back-coupled valve |23 functions as a blocking oscillator, and generates a positive pulse lasting for about 300 microseconds 'at-a repetitionfrequency -of say 1000 -cycles a second.` The crystal oscillator-|22 is coupled to the inner grid, and the blocking oscillator |23 to the suppressor grid, of a pentode |24, which acts as a mixen output consists of a train of waves at crystal frequency, lasting for 300 microseconds, followed by a quiescent period of 700 microseconds.

Theoutput from the pentode |24 is -applied to one of the Y-plates `of the range tube 28, and simultaneously triggers the time-base circuit 34 which supplies one ofthe X-plates.

The initial bias on the pentode |24 is set to prevent current fiow until both the inner and suppressor grids are simultaneously impulsed. This ensures accurate synchronization between the crystal-controlled oscillator |22 (which is constantly in operation) ,and the time-base 34, in spite of any slight variations in the frequency of the blocking oscillator. The result is a stationary-Wave trace on the screen, in which each wave is separated from its neighbour by a distance representing a range of a thousand yards. A selected wave-peak, say the vthird, is rst aligned with the vertical cross-wire on the screen, and the potentiometer handle 42 is then rotated to move the waves in succession across the screen, until the peak, say of the 13th wave, has replaced the third. The diierence between the initial and final readings on the scale of the potentiometer control should then represent 10,000 yards in actual range; otherwise -a-trimming condenser in the time-base circuit must be adjusted until this is the case. Once these -two adjustments have been made, the exponential law of the potentiometer and time-base automatically ensures the required alignment.

The cathode ray tubes are shown more clearly in Fig. 1l which also shows the interconnection and the relation to earth potential of the associated power packs, time base circuits and the potentiometer. The range tube 28 is provided with the usual defiecting plates |30, cathode |3|, grid |32, and anodes |33, |34, |35, the necessary operating potentials being derived from a. 3000 voit power pack |35 common to both range and bearing tubes. The usual control (indicated in the panel |31) are provided for focussing and regulating the brightness of the spot. In addition two other controls are Aused to eliminate spot astigmatism; one (not shown) applies a voltage simultaneously to the two Y-plates to produce an eiect similar to a cylindrical lens; the other, |38, produces an analagous and complementary effect by varying the voltage on the third anode |35. The cathode |3| and grid |32 are operated at 3000 volts negative and the focussing anode |34 at about 2500 volts negative. The rst and third anodes |33, |35 are strapped together and are at approximately earth potentional. The time-base circuits 32, 33, 34 and 36 for the range tube aremounted on the panel |39 and the timing condenser 44 and charging resistance 45 are mounted on the potentiometer panel |40 which carries the range potentiometer 40 and its guard resistance 54. The range potentiometer and time base are fed from the 4,000 volt and 250 volt power packs |4| and |42. The chassis of these power packs and of the panel |39 are insulated from earth, the minimum insulation resistance being 1000 megohms. unit (Fig. 2). The bearing tube is shown at 29 and itsassociated brightness and focussing controls in the panel |44. The bearing time base and bearing potentiometer 4| are fed from the 2000 volt power pack |45.

lThe apparatus now to be described with reference to Figs. 12-19 differs from that already described in that (1) means are provided for indicating the angle of elevation of the target, in addition to the angle of bearing, (2) the transmitter is designed to produce much shorter pulses lasting approximately one microsecond, (3) the receiver is designed to give a more positive and accurate indication of the position of minimum reception for determining the angles of bearing and elevation and also to indicate without ambiguity whether these angles are increasing or decreasing, and (4), in addition to the potentiometer control already described, signal selecting means are provided for ensuring that only the echo trace selected by the range tube observer can appear on the screens of the bearing and elevation indicating tubes.

The transmitter employs a modulator unit. controlling a master-oscillator, a radio frequency Lead |43 is connected to the calibrator amplifier coupling the oscillator to the transmitting aerials, and monitor tube and associated time base for supervising the performance of the transmitter. The general lay-out' of these components is similar to that shown in Fig. 1 and need not be described again.

The time taken for a wireless signal to travel two thousand yards is known to be approximately sii: microseconds, which is equivalent, when measuring distances by reflection, to a slantrange of one thousand yards. The exploring signal from the transmitter is radiated in pulses in order to allow the reflected signal or echo to be discerned duringthe quiescent period following each pulse, and this, in practice, sets certain limits upon the accuracy with which measurements can be made. During active periods of transmission the receiver is, in effect, out of action, so that a prolonged pulse would prevent observations from being taken at short range. For observations at long range, the duration of the pulse determines the accuracy with which it is possible to discriminate between two distant bodies in close proximity to each other. In a cathode ray indicator, for example, it xes the base-width of the echo-trace seen on thescreen of the tube, and therefore the point at which the traces begin to over-lap for a given difference in slant-range. When directional indications are required any overlap of the traces will also vitiate the necessary comparison of signal strengths.

'I'he power which can be developed by a thermionic oscillation-generator is largely determined by the maximum emission from its cathode. A valve with a thoriated lament, for instance, is capable of developing a peak output of kilowatts when activated intermittently for periods of the order of onemicrosecond.

From the above considerations it is clear that lconsiderable advantages are obtained by making the duration of each pulse as short as possible, and the master-oscillator illustrated in Fig. 12 and the associated modulator circuit illustrated in Fig. ,i3 are designed with this end in view.

l Referring to Fig. 12. the exploring signal is generated by two push-pull valves 200, 20|, which are of the thoriated-nlament or other known type capable of developing high power for short periods. A tuned Lecher-wire 202 forming the common anode circuit, is connected to the hightension supply through a choke 203 and a ballasting resistance 204.

To prevent overloading, the valves actually draw current during each signalling pulse from ycondensers 205, 208 which are 'connected from the mid-point of the resistance and choke to earth. The oscillator grids are similarly joined to a common tuned Lecher circuit 201, which is Ynormally held at a high negative potential but 1s periodically swept, at pulsing or repetitionfrequency, by a positive triggering voltage developed by the modulator unit shown in Fig. 13. The oscillator load inductances 208, 200 are connected across each cathode to earth, and are tapped through a transmission line including Vcondensers 2|0, 2|| to a pair of similar inductances 2|2, 2|3 forming the cathode input circuit of the radio frequency ampliiler. Each of the cathode inductances is made tubular to enclose one of the filament supply leads, the heating current passing to one iilament by the inside wire, then through one filament and along the outer tube to the next iilament, and back through the inner wire.

Y' effect ceases, and the condenser 23| :charges up to its original negative value, thus Both Lecher circuits of the osci.lator are tuned to approximately a quarter-wave and serve primarily' as phasing reactances. automatically floats into a condition of stability with respect to the supply voltages, the operating frequency ,being determined by the Lecher circuits, the inter-electrode capacities, and the cathode impedance. The working range extends from 50 to 85 megacycles, the cathode impedances being tapped to earth through condensers 2|4, 2|5 over part of this band.

When the grid circuit of the oscillator is impulsed from the modulator unit, both valves start to osclllate and rectified grid current simultaneously begins to build up across a condenser 2|6, which is shunted by a resistance 2|1. 'I'he value of the condenser is calculated to allow each pulse or train of signal waves to persist for approximately one microsecond, at the end of which time the negative voltage automatically quenches the oscillator. The normal bias is re-imposed before-the condenser can discharge through the resistance 2|'|, and the oscillator remains inactive until the onset of the next triggering impulse.

The radio-frequency amplifier consists of a pair of push-pull valves 2|8, 2|9 similar to those used in the oscillator, It has a common Lechertuned anode circuit 229 which draws current, as before, from condensers 22|, connected to earth between a ballasting resistance 222 and highfrequency choke 223. The grids are not tuned. They are normally biased to cut-01T through a choke 224 and are earthed through condensers 225. The cathode inductances 2l`2, 2|3 are similar to those of the oscillator unit, except that the coupling-line is taken to fixed points. In operation the input voltage from the oscillator oisets the initial grid bias on the amplifier and drives it for clear-cut periods of approximately one microsecond. The method of coupling employed has the advantage of automatically stabilising the amplifier, without the use of neutralizing or balancing condensers. In addition it transfers a proportion of the oscillator output direct to the aerial through the amplifier capacities, thus serving to increase efciency. The output from the amplifier is fed to the aerial system through adjustable tappings 226, on the anode Lecher wires.

A fraction of the high-frequency signal is tapped 01T from the feed-line through an attennation-line including the resistances 221, 228, and a delay network, to the signal plates of the monitor tube. Simultaneously locking impulses are taken 01T from a resistance 229 and fed through a concentric cable to synchronize the time-base circuit of the receiver.

Referring to Fig. 13, the modulator unit includes a back-coupled valve 230 arranged to produce a square-topped output wave at a repetition-frequency which can be varied between limits of say lfOO to 2000 cycles a second. To prevent deliberate jamming or interference, a slight wobble can be superposed on the selected repetition-frequency.

"The cathode of the valve 230 is earthed, and.

its grid is given an initial negative bias which leaks away at a rate determined by the timeconstant of the condenser 23| and resistance 232, until the valve starts to pass current, whereupon the back-coupling action through the transformer 233 accelerates the rise of the current to saturation. At this point, the back-coupling rapidly producing a periodic square-topped voltage at a The oscillator frequency which can be further adjusted either by varying the initial biasing condition, or 'ny altering the time-constant of the grid circuit including the resistances 232, 234. The output from valve 230 is fed flrst through a condenser 235 to an amplifier 236, and then through a condenser 231 to a second amplifier 238. Inits pas- -V sage through the circuits of these valves, the square-topped wave is converted into a sharp peak of positive voltage. This is applied to the grids of two cathode-follower valves 239, 240 arranged in parallel, the resulting change in cathode voltage being fed yfrom the resistances 24|, 242 to trigger the master oscillator.

The cathode bus-bar 243 is normally 100i) volts negative so that the mid-point of the resistances 24|, 242, and therefore the potential/on the grids of the valves 20|, 202 (Fig. 12) will be at the same potential until the valves 239, 240 start to conduct, whereupon it will suddenly rise by approximately 900 volts. A gas-filled discharge valve 244, in the cathode circuit of the valves 239, 249 acts as a flexible resistance to minimize the A effect of voltage fluctuations occurring between successive peaks, whilst a gas-filled diode 245 is.

provided to by-pass any voltage surge due to ash-over in the main oscillator unit.

To apply a frequency wobble to the rate at which control pulses are fed to the oscillator unit,

the grid and screen of a valve 246 are backcoupled to generate high-frequency oscillations,

which are first automatically converted into re.

laxation oscillations having a frequency determined by the values of the grid condenser 241 and leak resistance 248. In practice the second or relaxation frequency is arranged to be of the order of four cycles a second. Meanwhile a condenser 249 shunted across the anode and cathode of the valve 246 is charged through a resistance 250, and is discharged at the relaxation frequency. The saw-toothed voltage so produced is applied through a condenser 25| to the grid of a low-impedance cathode-follower valve 252, the resulting changes of cathode voltage being fed through a potentiometer 253, to vary t-he grid-bias and hence the repetition frequency of the valve 23D. The frequency wobble can, in turn, be adjusted by varying the setting of the potentiometer 253. A filter circuit formed by condensers 254, 255 and resistance 256, serves to bypass high-frequency oscillation in the output of the valve 246. The time base circuit of the monitor tube is synchronized with the outgoing pulses, by negative impulses from the anode circuit of the amplifier 236, which are taken off at the terminal 2.41. The forward stroke of the spot is made visible by means of positive impulses from the resistance 2148v which overcome a negative bias on the monitor tube grid. The back-stroke is thus automatically eliminated.

The pulsed exploring signal is radiated from one or the other of two alternative aerial systems, of which one is a single horizontal dipole 249 (Fig. 12) with a broadside figure-of-eight response curve, and is used for searching operations. The other aerial comprises an array of four horizontal dipoles, 250 which is backed by a screen of wire-netting and radiates a unilateral beam of high concentration in the direction of the target. For this' purpose the aerial array is rotated, to face the right direction, in accordance with information automatically transmitted by Selsyn or similar gearing from the bearing indi cator in the receiver. The dipoles are provided with impedance-matching stubs to which the feed-lines are adjustably coupled in accordance with particular signal frequency that is being transmitted. Telescopic end-pieces are also fitted, both to the dipoles proper and to the matching stubs, to enable them to be adjusted for working over a frequency range of say 55 to 85 megacycles.

The principal circuits of the receiver are shown schematically in Fig. 14. Before describing the circuits in detail, an outline will be given of the more important features.

The slant-range of the target is found by measuring the time-interval between an outgoing pulse and its reflected echo, as indicated by the displacement of the echo trace on the fluorescent screen of a cathode ray tube This displacement is measured by means of a calibrated potentiometer 252 which is used to bring the echotrace to the centre of the screen, thereby allowing the nominal displacement of the trace to be measured in terms of the control voltage required to centre it. By means of the ganged potentiometer 253 this controlv is applied simultaneously to the cathode ray tubes 254, 255 which indicate the angles of bearing and elevation respectively, so that when any selected echo-trace is centered, say on the range tube, the counterpart echo-traces are simultaneously brought to the centres of both the other tubes. This method of co-ordinated potentiometer control is the same as that previously described with reference to Figs. 3-5.

The cathode ray tube 254 is used to indicate the bearing in azimuth of the distant body, by swinging an aerial system 256 comprising two horizontally separated dipoles until the setting is reached at which the output has a minimum value. For this purpose the signals received'on the bearing aerial are combined first in phase and then in phase-opposition with those received on the range aerial 251 through a continuously driven switch 258 so as to produce the so-called cardioid response. The 'required bearing is shown when the trace on the screen remains at constant amplitude, because no output is then being received from the bearing aerial. Meanwhile any deviation to port is clearly differentiated from a deviation to starboard so that once a bearing has been obtained, subsequent changes in bearing oi' the body under observation can be followed without ambiguity. i

A third cathode ray tube 255 is used to indicate the elevation of the distant body. Here the reilected signal is received simultaneously on two aerials 260, 26|, which are displaced inthe vertical plane, and are coupled to a goniometer 262, the angle of elevation being shown by the setting of the goniometer which gives zero or minimum output. Here also the signals received by the elevation dipoles are combined irst in phase and then in phase opposition with those received on the rangedipole by means of a continuouslyrotated switch 259, whereby a similar cardioid effect is produced to allow increases in the angle of elevation of the body to be'clearly distinguished from decreases in this angle.

Special provision is made to facilitate the necessary comparison of signal strengths on the bearing and elevation tubes. In the irst place a signal selecting device 262 which is under the control of an auxiliary potentiometer 263 ganged to the main potentiometer unit, is used to prevent any signal-trace, other than the one already centered, from appearing on the screens of the bear- A goniometer.

ing and elevation tubes, automatic volume control being applied to the traces shown. In the next place the sum and difference signals produced by the action of the reversing switch 258, 259 are separated from each other along the time-base and are presented in different colours on the screens of both tubes.

A single horizontal dipole 251 serves as the range aerial and is mounted half a wavelength above ground in the same vertical plane as a pair of similar dipoles 258 which are spaced apart horizontally to give a directional response in azimuth. The maximum forward lobe of the figureof-eight response of the range dipole 251 is arranged to coincide with the forward minimum or crevasse in the buttery-wing response of the bearing dipoles 256. The latter are connected, in opposition, to the aerial reversing switch 258 through a phasing-box 264 which permits' the bearing signal to be brought into phase with the signal from the range aerial, should the bearing dipoles not be broadside-on'to the target.

The elevation-finding aerial consists of two horizontal dipoles 260, 26| which are spaced apart vertically and are connected to the two fixed coils of a goniometer, the search-coil of which is rcoupled to the aerial reversing switch 256. The signals received by the two dipoles are in the same phase (or in phase opposition) but differ in amplitude to an extent which depends upon the elevation of the reflecting body. Moreover, if the upper and lower dipoles are kept at the same relative heights, interms of the working wavelength, the amplitude ratio of the received signals remains constant for different wavelengths. The variation of signal ratio does not, however, follow a straight-line law with variation in elevation of the body under observation so that it is necessary to introduce a correcting factor in order to simplify calibration. For this purpose, the relative spacing and height of the dipoles are rst chosen to give a signalratio curve which not only lends itself to uniform calibration but also shows high sensitivity over Va range of angles extending say from 10 to 80 from the horizontal. These conditions are found to exist when one dipole is set approximately one wavelength above ground and the other approximately half a wavelength above it. A correcting cam having the same contour as the optimum signal-ratio curve is therefore interposed between the shaft of the search-coil of the goniometer and a control dial so as to allow the latter to be uniformly calibrated in degrees of elevation.

In order to prevent interaction between the bearing and elevation dipoles, they are fed by transmission lines which are an integral number of half waves long, s0 that both aerials work into a high impedance. Owing to imperfect reflection from the ground some difference in phase may arise between the two elevation dipoles, so that they do not completely balance out across the In this case equality on both sides of the aerial reversing switch can be obtained by turning the search coil until the residual signal is in phase-quadrature with the signal from the range dipole. When searching for a .distant body, the top elevation dipole is preferably used in place of the range dipole and is provided with a reflector t`o determine sense. In these circumstances the stand-by switch is opened, and the goniometer control is adjusted so that the search-coil is coupled only to the fixed coil of the upper dipole.

A11 the dipoles are itted with adiustable stubs for matching the impedance oi the feed-lines t that of the aerial. and with telescopic end-pieces to tune the aerial to diii'erent frequencieawithin say a working range of 55 to-85 megacycles.

A switch 265, driven by the aerial reversing I switches 258, 258 applies a momentary voltage to the sweep deecting plates of the bearing and elevation tubes 25d, '255 in order to separate the trace produced by the addition of the range and bearing laerial voltages from the trace due to their dlierence on the tube 28d, and to separate the corresponding traces on the tube 252. This separation facilitates the necessary comparison of signal strengths. The aerial reversing switch also drives a stroboscoplc disc 258 'tted' with dierently-coloured windows, which rotates in front of both tubes and is so synchronized that the screen of the bearing tube is only visible when the bearing aerial is coupled to it, the elevation screen then being obturated and vice versa. a further aid to the observer the coloured windows are arranged so that the summation signalson both tubes 4are shown through the red whilst difference signals appear only through the grew windows of the stroboscopic disc.

The three aerial systems are connected to a so arranged that the timing of the impulses thus lals common signal receiver comprising a radio frequency ampliiier 2t1, mixer 208, local oscillator 269, intermediate frequency amplifier 212, second detector 21! and video output stages 212, 213, connected to the signal plates oi' the range indicating tube 258 and the bearing and elevation indicating tubes 25d, 255. The amplitude of the output Awill very cyclically, representing'in turn the output of the range aerial plus the difierential output of the bearing aerial, the range aerial output plus the differential output of the elevation aerials, the range aerial output minus the diierential output of the bearing aerials, and the range aerial output minus the diijerential output of the elevation aerials. Thiscyclical iiuctuation of the amplitude of the echo trace on the range tube screen is of little importance since it is the position of this trace which has to be measured and, in any event, the utuation tends to disappear when the bearing and elevation of the selected target have been properly obtained. since the dierential output of t le range and bearing aerials is then substanti Y y zero. Owing to the action of the stroboscopic disc 282 and the separating switch 255 already explained, the variations in output are visible as four separate and distinct traces on the screen of the bearing and elevation tubes.

A time base generator 214v for the range tube and a common time base generator 215 for the bearing and elevation tubes provide the necessary sweep voltages and are syncliionized' with the transmitter by means of a. locking amplifier 218 anda multivibrator 211. A third time base generator 218, also synchronized by the multivibrator is provided for the signal selecting circuit 262. This comprises a valve which is normally cut-oi and has the initial potential between its grid and its earthed cathode variable over a wide range by means of a potentiometer 283 ganged to the potentiometer-S 252, 253.' The sweep voltage from the generator 218 is applied to the grid. so that each time the rising voltage on the grid passes through earth potential and equals the l cathode voltage the valve suddenly conducts.

The time at which this occursdep'ends upon the setting of the potentiometer 262 and matters are the echo signal pulses producing the echo trace.

which is centred on the range tube screen by the action of the potentiometer 252. These signal selecting impulses are utilized to release or intensifyy the electron beams of the tubes 254, 2654 so that only the counterparts of the echo trace centred on the range tube screen can appear at f'ull brightness on the screens of the bearing and elevation tubes. The signal selecting impulses are also utilized to condition an automatic gain control circuit 218 for operation, whereby automatic gain control of the selected signal and no other is applied to certain stages of radio frequency amplier 281. Manual gain control is applied to this amplifier by means of the resistance 280. Reference 28| indicates a valve which is arranged to release the electron beam of the range tube during the forward sweep of the spot whilst 282 is a calibrator circuit which is utilized in a similar manner to that previously described.

Details of the signal receiver are shown in Fig. 15 and 16. The radio frequency ampliier 281 has five stages of which the first two 283, 288 and the last 285 are shown in Fig. 15. The stages are resistance capacity coupled and have tuned grid circuits connected by ganged controls. y

Automatic volume controlis applied to the grids of the third, fourth, and fth stages. A diode 258 serves as the mixer, the anode being coupled to the last stage 285 and the cathode to the local oscillator 269, the grid of the latter being separately tuned by a condenser 286. The resulting beat frequency is fed to the input of the intermediate-frequency amplier through a choke 281 and a band-pass circuit formed by two tuned circuits 208, 288 and 290, 29| (Fig. 16) which are coupled together by the screened connecting lead 282.

To eliminate heterodyne interference a lter circuit isv switched across the input to the intermediate-frequency amplifier (Fig. 16). It in cludes a. resistance 283 in series with a condenser 298, both shunted by a pair of variable condensers 295 which are, in turn, earthed through an inductance 296. The resistance 293 is separately earthed by a pair of condensers of which 291 is variable. The illter is tuned to the frequency to be cut out by varying the condensers 295, and

complete elimination is thensecur'ed by adjust- A circuit of the rst two stages is provided to reboth the bearing and elevation tubes 254, 255 and as shown in Fig. 17 (b) a.- separate discharger valve 305, condenser 306 and resistance 301 are used for the range tube 25|. An auxiliary timebase, shown in Fig. 17 (c) and comprising the discharger valve 308, `condenser 309 and resistance 3|0, is also provided for the signal-selecting device.

The potentiometer control unit already described comprises electrically-symmetrical limbs, such as 253, 252 (Figs. 17 a, b) which are branched across the supply voltages 3| I, 3|2 of the time-base circuits and are provided with ganged sliders 3|3, 3M. A selected echo-trace is brought to the centre cf the range tube 25| by setting the slider 3|4 sothat a suitable biasing voltage is applied to one of the deflecting plates of the tube. The slant-range of the distant body producing the selected signal-trace can then be read off directly in terms of the applied biasing voltage on the calibrated scale of the potentiometer. The electrical symmetry of the potentiometer, and the common movement of the ganged sliders 313, 3M automatically brings the same selected signal trace simultaneously to the centre of all three tubes. The use of leakageguards has been described with reference to Fig. 5, but in the present example they are of rather different construction. Referring to Fig. 17 (a) the guard resistance described in Fig. 5 is replaced by two valves 3I5, 3|6 connected in series across the power supply 3|I. The valve 3|0 is a pentode or other valve having a constant current characteristic and serves in effect as a sensitive high resistance in the cathode circuit of the valve 3I5. The latter accordingly acts as a cathode-follower and the voltage on its grid follows that on its cathode under all working conditions. As before the power supply floats, the grid of 3|5 being connected to the slider 3|3, whilst its cathode is the only earthed point in the system. Any leakage current from the high potential parts of the system to earth will ow through 3I5, but the grid connection automatically keeps 3|3 approximately at earth potential for all settings. The microammeter 3|1 in the earthed lead serves to indicate the presence of any excessive leakage. Similar components 388, 3|3, 320 are provided for the potentiometer 252 in Fig. 17 (b) An auxiliary branch 253 (Fig. 17o) of the potentiometer unit is provided in combination with the auxiliary time-base circuit 308, 309, 3I0 for the purpose of actuating the signal-selecting device whereby all signal traces are excluded from the screens of the bearing and elevation tubes other than the particular trace which has already been centered by the potentiometer control. The manner in which this is done will be described in detail in Fig. 19. For the moment it is important to note that the cathode of the cathode-follower valve 32| of the leakage guard is earthed,that its grid is therefore approximately at earth potential, and that the slider 322 is ganged to the other sliders 3|3, 3|4 of the potentiometer. It should also be stated that the resistance 323 is for all relevant purposes the equivalent of the constant-current valves 3|6, 3|9 in the other guard circuits.

The time-base circuit for the range tube is shown in detail in Fig. 18. synchronizing impulses are fed` from the transmitterthrough a concentric cable, and are applied to the cathode of a locking ampliiier 324. The anode of this valve is connected to the grid of the discharge valve 305, Fig. 17 (b), and to the anode of one of two valves 325, 326 which form a crosscoupled relay or multivibrator. Initially the valve 320 is conducting and keeps the valve 325 non-conducting by the bias developed across the common cathode resistance 321. The negative impulse from the locking amplii'er 324 trips the relay so' that the valve 320 is paralyzed and the valve 325 startsto conduct. This persists until the charge built upon the condenser 320 leaks away, whereupon-the relay reverts to its initial condition. The result is that the original voltage impulse from the locking amplier is prolonged for a period of approximately 300 microseconds. This prolonged pulse is also applied to the grid of the time-base discharge valve 305, and also to a valve 28I which passes a positive pulse to the negatively-biased.control grid of the range tube 25| and releases the electron stream during the forward sweep oi the spot.

The time-base circuit for the bearing and elevation tubes is similar to the one used for the range tube. It is synchronized by the multivibrator 325, 32B. The sweeping voltages are however applied to traverse the spot vertically instead of horizontally across the screen.

To ensure clear focussing it is desirable that the voltage on both sweep deflecting plates should be approximately the same as the voltage on the earthed anode of each cathode ray tube. For

this reason, the potentiometer unit, and each' time-base Icircuit and power supply pack, are all carefully insulated, the control slider of the p0- tentiometer being the ony earthed point in each system. Actually, as already described, the cathode of the valves 3I5, 3I8, 32| (Figs. 17 a, b, c) in the guard circuits are earthed, and the sliders are taken to the grids of these valves, but since these valves function as cathode-followers their grids and cathodes keep in step, and the voltage on the sliders and on the sweep deilecting plates of the tubes .is therefore always substantially at earth potential.

Fig. 19 shows the signal-selecting and automatic gain control circuits, and also the cathodefollower 32| forming part of the potentiometer guard circuit of Fig. 17 (c) The signal-selector time-base condenser 309 (Fig. 17o) is charged from the G-volt supply through a resistance 3|0 and is discharged by the valve 308 which is synchronized by impulses from the multivibrator 325, 326 (Fig. 18). Signal selection begins when voltage from the charging condenser 309 sweeps the grid of a valve 330, Fig. 19, past earth potential. The time at which this occurs during each cycle of the sweep voltage can be varied by means of the potentiometer 263 (Fig. 17 (ci) since the setting of this potentiometer ixes the starting value of the voltage sweep in relation to earth potential, i. e. the voltage time curve of the condenser 309 will be shifted bodily up and down with respect to earth potential by varying the setting of the potentiometer 263. Since the latter is ganged to the potentiometer 252 for the range tube the act of operating 252 to centre a trace on the range tube automatically ensures that the time at which the sweep voltage passes earth potential coincides with the arrival of the selected echo. Consequently the valve 330 conducts, each time the spot passes the centre of the screen of the range tube. The resulting variations in voltage are applied from the anode of the valve 330 through a condenser 33| to the grid 

